1. Field of the Invention
The present invention relates to a PET apparatus which can visualize behaviors of trace substances labeled with positron emitting isotopes (RI sources).
2. Related Background Art
PET (positron emission tomography) apparatus are apparatus which can visualize behaviors of trace substances within a living body (subject) having an RI source administered therein by detecting a pair of photons occurring as an electron/positron pair annihilation and flying in directions opposite from each other. A PET apparatus is equipped with a detecting unit having a number of small-size photon detectors arranged about a measurement space in which the subject is placed, detects and stores photon pairs occurring as electron/positron pairs annihilation by coincidence counting, and reconstructs an image indicative of a spatial distributions with respect to the frequency of generation of photon pairs in the measurement space, on the basis of the stored number of coincidence-counting information items, or projection data. The PET apparatus play an important role in the field of nuclear medicine and the like, whereby biological functions and higher-order functions of brains can be studied by using it. Such PET apparatus can be roughly classified into two-dimensional PET apparatus, three-dimensional PET apparatus, and slice-septa-retractable type three-dimensional PET apparatus.
FIG. 1 is a view for explaining the configuration of a detecting unit of a two-dimensional PET apparatus. This drawing shows a cross section obtained when the detecting unit is cut along a plane including the center axis. The detecting unit 10 of the two-dimensional PET apparatus has detector rings R1 to R7 stacked between shields 11 and 12. Each of the detector rings R1 to R7 has a plurality of photon detectors arranged like a ring on a plane perpendicular to the center axis. Each photon detector is a scintillation detector in which a scintillator such as BGO (Bi4Ge3O12), for example, and a photomultiplier tube are combined together; and detects photons reaching there after flying from the measurement space including the center axis. Disposed inside the detector 10 are slice septa 20. The slice septa 20 comprise six ring-like shield plates S1 to S6 disposed at respective positions between neighboring detector rings. Due to the collimating action of the slice septa 20, thus configured detecting unit 10 of the two-dimensional PET apparatus can detect only photon pairs flying from directions forming an angle of about 90 degrees with respect to the center axis. Namely, the coincidence-counting information, i.e., two-dimensional projection data, obtained and stored by the detecting unit 10 of the two-dimensional PET apparatus is limited to that obtained by a pair of photon detectors included in the same detector rings or detector rings adjacent each other (or very close to each other). Therefore, the two-dimensional PET apparatus can efficiently eliminate scattered photons in which photon pairs are generated at positions outside the measurement space, and can easily carry out attenuation correction and detector sensitivity correction with respect to the two-dimensional projection data.
FIG. 2 is a view for explaining the configuration of a detecting unit of a three-dimensional PET apparatus. This drawing also shows across section obtained when the detecting unit is cut along a plane including the center axis. The detecting unit 10 in the three-dimensional PET apparatus is configured similarly to that in the two-dimensional PET apparatus. However, the three-dimensional PET apparatus is not equipped with slice septa. Thus configured detecting unit 10 of the three-dimensional PET apparatus can detect photon pairs coming from all the directions. Namely, the coincidence-counting information, i.e., three-dimensional projection data, obtained and stored by the detecting unit 10 of the three-dimensional PET apparatus can be that obtained by a pair of photon detectors included in any detector rings. Therefore, the three-dimensional PET apparatus can detect photon pairs at a sensitivity higher than that in the two-dimensional PET apparatus by about 5 to 10 times.
FIGS. 3A and 3B are views for explaining the configuration of a detecting unit of a slice-septa-retractable type three-dimensional PET apparatus. These drawings also show across section obtained when the detecting unit is cut along a plane including the center axis. The detecting unit 10 in the slice-septa-retractable type three-dimensional PET apparatus is configured similarly to that in the two-dimensional PET apparatus. However, the slice septa 20 in the slice-septa-retractable type three-dimensional PET apparatus can be retracted into a shelter space provided on the side of a shield 12. Namely, the slice-septa-retractable type three-dimensional PET apparatus is equivalent to the two-dimensional PET apparatus when the slice septa 20 are positioned inside the detector rings R1 to R5 (FIG. 3A), and is equivalent to the three-dimensional PET apparatus when the slice septa 20 are in the shelter space (FIG. 3B). Therefore, the slice-septa-retractable type three-dimensional PET apparatus is used as one of the two-dimensional PET apparatus and three-dimensional PET apparatus depending on the aimed purpose.
In the conventional PET apparatus mentioned above, however, the two-dimensional PET apparatus is hard to detect photon pairs with high sensitivity since it detects only the photon pairs coming from directions at an angle of about 90 degrees with respect to the center axis. On the other hand, the three-dimensional PET apparatus is hard to efficiently eliminate scattered photons in which photons generated in the space outside the measurement space are scattered, whereas its scatter correction, attenuation correction, and detector sensitivity correction are difficult or complicated, whereby favorable images are hard to reconstruct.
Since the slice-septa-retractable three-dimensional PET apparatus acquires two-dimensional projection data and three-dimensional projection data upon separate measurement operations, it is hard to overcome the respective problems inherent in the two-dimensional PET apparatus and three-dimensional PET apparatus mentioned above at the same time. The apparatus configuration may become complicated and expensive.
In order to overcome the problems mentioned above, it is an object of the present invention to provide a PET apparatus which can simultaneously acquire two-dimensional projection data and three-dimensional projection data, thereby enabling photon pair coincidence counting with high-sensitivity, effective scattering correction, and the like.
The PET apparatus in accordance with the present invention comprises (1) a detecting unit including a plurality of sets of detector rings, each detector ring comprising a plurality of photon detectors disposed on a plane perpendicular to a center axis, each photon detector detecting a photon coming from a measurement space including the center axis, the plurality of sets of detector rings being stacked in a direction parallel to the center axis; (2) slice septa disposed rotatable about the center axis on the measurement space side of a part of the plurality of photon detectors constituting each of the plurality of detector rings, the slice septa transmitting therethrough only a flying photon substantially perpendicular to the center axis; (3) slice septa position determining means for determining, when a pair of photon detectors in the photon detectors included in the detecting unit detect a photon pair, whether or not the slice septa exist on the measurement space side of at least one of the pair of photon detectors; (4) two-dimensional projection image storage means for storing, when it is determined by the slice septa position determining means that the slice septa exist on the measurement space side of at least one of the pair of photon detectors, coincidence-counting information of the photon pair obtained by the pair of photon detectors; (5) three-dimensional projection data storage means for storing, when it is determined by the slice septa position determining means that the slice septa do not exist on the measurement space side of any of the pair of photon detectors, coincidence counting information obtained by the pair of photon detectors; and (6) image reconstructing means for reconstructing, according to three-dimensional projection data generated by the two-dimensional projection data storage means from coincidence-counting information stored thereby and three-dimensional projection data generated by the three-dimensional projection data storage means from coincidence-counting information stored thereby, an image indicative of a spatial distribution of a frequency at which photon pairs occur in the measurement space.
In the PET apparatus, when a photon pair coming from the measurement space is detected by a pair of photon detectors in the detecting unit, it is determined by the slice septa position determining means whether or not the slice septa exist on the measurement space side of at least one of a pair of the photon detectors. This determination is carried out according to the rotational position of the slice septa detected by angular encodor, for example. If it is determined by the slice septa position determining means that the slice septa exist on the measurement space side of at least one of a pair of photon detectors, then the coincidence-counting information of photon pair obtained by the pair of photon detectors is stored by the two-dimensional projection data storage means. If it is determined by the slice septa position determining means that no slice septa exist on the measurement space side of any of them, then the coincidence-counting information of photon pairs obtained by the pair of photon detectors is stored into the three-dimensional projection data storage means. Then, according to the three-dimensional projection data generated by the two-dimensional projection data storage means from the coincidence-counting information stored thereby and three-dimensional projection data generated by the three-dimensional projection data storage means from the coincidence-counting information stored thereby, the image reconstructing means reconstructs an image indicative of a spatial distribution of a frequency at which photon pairs are generated in the measurement space. Thus, the two-dimensional projection data and three-dimensional projection data are simultaneously obtained in one measurement procedure. Therefore, when images are reconstructed by simultaneously acquiring the two-dimensional projection data and three-dimensional projection data as such, photon pairs can be detected with high sensitivity, and scatter correction and the like can be carried out.
The image reconstructing means may reconstruct the images using components having lower spatial frequencies in the two-dimensional projection data and components having higher spatial frequencies in the three-dimensional projection data. In this case, the three-dimensional projection data can favorably be obtained by detecting photon pairs with high sensitivity, and scatter events can effectively be corrected in a favorable manner. Also, scattering can be corrected in the projection data having large angles of inclination.
The PET apparatus may further comprise correction means for correcting the image reconstructed by the image reconstructing means according to the two-dimensional projection data stored in the two-dimensional projection data storage means by providing a rod-shaped calibration source parallel to the center axis at the measurement space side of the slice septa. In this case, detector sensitivity correction and attenuation correction are carried out favorably, whereby a favorable reconstructed image is obtained.